1. Field of the Invention
This invention concerns starting cathodes of copper strip for copper electrolysis and a method of producing these starting cathodes.
2. Prior Art
In copper electrolysis, the copper raw material which is produced by smelting metallurgy and has a purity of 99.0% to 99.8% is dissolved at the anode primarily as Cu2+ and is deposited at the cathode as pure copper (high grade) with a high selectivity. Either thin substrates produced by electrolysis (starting sheets) or permanent cathodes of high-grade steel are used for cathodic deposition. The electrolytic copper produced by copper electrolysis has a purity of 99.95% to 99.99% and is used to manufacture semifinished products from this metal and its alloys.
The substrates used to produce starting sheets either consist of cold-milled polished copper, high-grade steel or titanium. The starting sheets are produced in so-called starting sheet baths. After electrolytic deposition on the starting sheets in a recurring rhythm of 24 hours, the deposited layers are separated either manually or by an automatic stripping machine. These sheets, which are known as substrates and correspond approximately in length and width to the anode and cathode dimensions are 0.5 to 1 mm thick and weigh approx. 4 to 7 kg. Preparation for starting sheets includes essentially cutting uneven, cracked edges, which may optionally be necessary, straightening and applying two mounting strips (xe2x80x9cearsxe2x80x9d of cut substrates or milled copper strip) to the cathode rod by means of an automatic riveting machine. This xe2x80x9cstarting sheetxe2x80x9d production technology is out-of-date and is no longer economical. This has been a problem in the copper industry for a long time, because the demand for high-grade steel plate and the required high quality standard for starting sheets leads to high costs with regard to the cost of acquisition as well as the labor expenditure, power consumption and time consumed as well as leading to a high rate of waste in starting sheet production.
For example, the starting sheet will usually have a fixed dimension which is limited by the size of the electrolysis bath. Industrially, however, it is important for the starting sheet anode to have an optimum size because of the high costs of labor and energy in anode production and the reprocessing of anode residues after electrolytic metal deposition. However, the anode must have almost complete and uniform coverage of the starting sheet, so that in practice, the size of the anode is adapted to the size of the starting sheets and other process variables in order to lower production costs for starting sheets. This usually leads to the production of two types of anodes which differ in geometry:
starting sheet anodes and
production anodes.
Starting sheet anodes also tend to bend or roll up and do not hang straight in the production bath due to lack of uniformity in thickness and due to the production method (stripping the substrates away from the starting sheet). Disadvantages include the unavoidable cracked edges due to the manufacturing process plus the fact that a smooth surface is not always guaranteed.
The known consequences include short-circuits leading to a low current efficiency and a reduction in production volume, associated with a negative effect on cathode quality.
Permanent cathodes made of high-grade steel are used with the copper refining process that has been introduced in the meantime under the name xe2x80x9cISA process.xe2x80x9d Copper is deposited on these cathodes usually over a period of seven days and is separated mechanically in the form of sheets by means of an automatic stripping machine.
The ISA process is very expensive and leads to high production costs for the refined copper. In addition, large inventories of high-grade steel plate is necessary for the ISA process, leading to additional storage and warehousing costs. Another disadvantage of the ISA process is that the starting sheets needed for electrolyte regeneration for decopperizing electrolysis must usually be purchased from third-party operations.
The profitability of copper electrolysis depends essentially on the quality of the copper sheets used as the starting cathodes and their production costs.
International patent WO 97/42360 describes a method of producing copper cathode starting sheets where refined copper is smelted and then processed through continuous casting and milling methods to form strips 0.635 mm to 1.778 mm thick (0.025 to 0.070 inch), which corresponds to a 25% to 98% reduction in the starting material thickness. This requires casting in a horizontal position and also conveying the sheets in a horizontal position to the reducing plant, a roll mill. The cast strip obtained in the first process step should have a thickness of 5.08 to 38.1 mm (0.2 to 1.5 inch). In addition, it is essential for the milled strip not to be rolled or otherwise deformed during or after milling to rule out the possibility of the so-called memory effect (a horizontal curvature of a few mm) in use as starting sheets. The memory effect is the main cause of short circuits that occur during copper electrolysis.
The starting sheets are cut out of the milled strip and fabricated in a known way for the electrolysis process.
This proposed procedure for producing copper cathode starting sheets is very expensive due to the high installation costs. The installation is designed for the usual width dimensions of the starting cathodes and is intended only for production of starting cathodes. Based on approximately 200,000 tons per year and the annual demand of approximately 35 tons of starting cathodes per year for utilization of capacity. Therefore, the production costs of starting cathodes is very high. In addition, this method is limited to processing refined copper. Another disadvantage is that the milled copper strip for manufacturing the starting cathodes must not be rolled or otherwise deformed. Consequently, the milled copper strip cannot be rolled up into a coil but instead must be stored and transported only in the form of prefabricated cut sheets, or the milled sheets must be processed directly to starting cathodes within the production line. Another fear is that due to the deformation caused by the milling operation, it is impossible to rule out a memory effect of the starting cathodes during copper electrolysis. The aforementioned publication also does not mention any results documenting that no memory effect occurs in use of the starting cathodes produced in this way.
The object of this invention is to create starting cathodes from copper sheet for copper electrolysis that will rule out any memory effect during copper electrolysis, will permit a high production output of electrolytic copper and can also be produced from directly shaped copper sheet material in the form of a coil.
Furthermore, a suitable method of manufacturing the starting cathodes is to be created which is also suitable in particular for processing conventionally manufactured copper strip.
Through the process step of subjecting the milled copper sheet to an additional soft annealing process which is essential to this invention, it is possible to eliminate the memory effect which otherwise occurs when using starting cathodes in electrolysis. Therefore, there are far fewer short circuits during copper electrolysis and current efficiency is higher. Copper electrolysis can thus be performed more efficiently and with a higher cathode power. It is also advantageous to use the grades of copper according to DIN standards 1708, 1787 and 17670 which have a higher concentration of metallic impurities in comparison with electrolytic copper and refined copper. It has surprisingly been found that when using starting cathodes made of these grades of copper, the quantity of electrolytically deposited copper with a higher purity is greater. In comparison with the starting sheets used according to International Patent No. WO 97/42360 which must have a minimum thickness of at least 0.635 mm, experiments have shown that when using milled and soft annealed starting sheets, the sheet thickness can be reduced to a level of less than 0.5 mm, with 0.3 mm being the lower limit. In comparison with thicker starting sheets, this reduces the cost of materials and also makes it possible to use a larger number of starting cathodes in the electrolysis bath. This is possible in particular only because the milled and soft-annealed starting sheets do not lead to a memory effect. The incidence of short circuits has been greatly reduced by using the starting cathodes according to this invention in copper electrolysis, and a current efficiency of 98% to 99% has been achieved.
Due to the lower thickness of the starting sheets and their lower weight, the thickness of the copper sheet for ear strips can also be reduced to preferably 0.3 to 0.5 mm.
The stated strength of the copper sheet of 210 to 240 N/mm2 is achieved by aftertreatment on a skin pass mill stand, for example.
Soft annealing of the milled copper sheet is performed at furnace temperatures of 700 to 750xc2x0 C., preferably at 720 to 750xc2x0 C., with the furnace temperature being reduced from 750xc2x0 C. to 720xc2x0 C. in the direction of passage. The speed of conveyance of the copper sheet through the furnace depends essentially on the sheet width and sheet thickness. For starting sheets for starting cathodes with a width of 930 mm and a thickness of 0.3 to 0.8 mm, this amounts to 20 to 55 m/min. Various technical options are available for performing the soft annealing process. The copper sheet can be produced in a traditional casting and milling installation and wound up as a coil. Then in a separate installation, the milling hard copper sheet is uncoiled, soft annealed in an annealing furnace, treated in a downstream degreasing and pickling unit (removing scale and oxides) and straightened in a straightening and partitioning installation and cut to the required length of 840 to 1250 mm. In this embodiment, pass milling of the copper sheet may be omitted. Then the ears are riveted on by means of a riveting and straightening machine and the contact rods are mounted. In a subsequent adjusting unit, the starting cathodes are separated, sorted and suspended in the prepared receptacle for the crane for suspending in the electrolysis bath. It is an important advantage that no separate installation is necessary for producing the starting sheets but instead this process starts from a soft annealed copper sheet produced by an essentially known method and optionally ordered from a third party.
This also applies to another variant according to which the mill-hard copper sheet is soft annealed while still in the casting and milling installation and then it is available as a soft annealed copper sheet in the form of a coil for further processing to produce starting cathodes. This coil is then uncoiled to produce the starting cathodes and supplied to the straightening and partitioning installation. Further processing then takes place as described above.
Furthermore, there is the possibility of manufacturing starting cathodes within one manufacturing line, and then the process steps of winding up and unwinding the coil of milled and soft annealed copper sheet may be omitted. Soft annealing of the milled copper sheet can be performed in an annealing furnace with a vertical or horizontal design. Before soft annealing, the copper sheet should be degreased, brushed, rinsed with water and dried. After annealing, it is expedient to pickle and neutralize the cooled copper sheet.